This invention is directed toward the field of switching systems. More specifically, the invention is a switching system which has a zero differential between sensing circuit operate and release points.
Sensors, such as Hall elements, are often used to trigger a switching system. An example of a switching system is where a Hall element and a magnet are used to create a proximity switching system. When a magnet is placed in close proximity to a biased Hall element, a voltage difference between two sensing terminals of the Hall element is produced. The magnet may be mounted on an apparatus which periodically moves towards and away from the Hall element. This periodic movement of the magnet produces a periodic magnetic field intensity at the Hall element and consequently a periodic voltage difference between sense terminals of the Hall element. This Hall element output voltage bears a linear relationship with the intensity of the magnetic field.
Because the output voltage of the Hall element is related to the distance of the magnet from the Hall element, the Hall element-magnet apparatus may be used for proximity switching. Often a voltage comparator circuit includes a voltage comparator in which positive feedback is used to accomplish switching. The voltage comparator has inverting and noninverting input terminals and produces a high output if the voltage difference across the terminals is positive, and a low output if the voltage difference across the input terminals is negative. A comparator circuit which is made of a voltage comparator with a feedback path, produces the same digital output, but due to the feedback, the switch from a low to high output occurs at a preselected operating voltage (V.sub.OP) and a switch from a high to a low output occurs at a different preselected release voltage V.sub.RE. V.sub.OP is the voltage at the sensor terminals at which the voltage across the input terminals of the voltage comparator equals zero, when the output of the comparator circuit is at a low state. V.sub.RE is the voltage at the sensor terminals at which the voltage across the input terminals of the voltage comparator equals zero, when the output of the voltage comparator circuit is at a high state. The difference between V.sub.OP and V.sub.RE is known as the differential.
Since the output voltage of the Hall element is related to magnetic field intensity, it is also possible to talk about a B.sub.OP and a B.sub.RE. B.sub.OP is the magnetic field intensity which causes the Hall element to produce a voltage of V.sub.OP across its terminals. B.sub.RE is the magnetic field intensity which causes the Hall element to produce a voltage of V.sub.RE across its terminals.
If an electromechanical pressure sensor were used, P.sub.OP would be the pressure at which the voltage across the sensor terminals is V.sub.OP. Accordingly, P.sub.RE is the pressure at which the voltage across the sensor terminals is VRE.
Some switching systems are based on the flow of a fluid, such as hydraulic or pneumatic systems. In these systems, a valve operates as the equivalent of the voltage comparator. When a positive difference in pressure is present between first and second inlets, fluid flow through an outlet is enabled. When a negative pressure difference is present between the first and second inlets, fluid flow through the outlet is prevented. Feedback is possible here as well by allowing fluid flow between the outlet and one of the inlets. This produces a differential between operate and release points as well.
In some applications, it is desirable to have the switching apparatus "turn on" at the same sensor input level as the sensor input level at which it turns off, or in other words have a zero differential system. For a Hall element switching system this can be expressed as B.sub.OP =B.sub.RE. Yet, when a single comparator circuit has an operate point equal to its release point, (no hysteresis), the comparator circuit is unstable. The comparator circuit's output, when the operate point equals the release point, V.sub.RE may oscillate between high and low output levels when the input voltage is in the vicinity of the operate or release points. Therefore this cannot be a solution to the problem.
Thus it is the object of the present invention to provide a stable switching system where the operate point is the same as the release point and wherein the system does not oscillate between high and low outputs when V.sub.OP is near V.sub.RE.